Advanced Cell Biology

• proteins cannot leave the cell if they are translated in the cytoplasm
• all proteins that are secreted out of the cell have to begin in the ER
• Why are some proteins translated in the cytoplasm while others are translated in the ER?

  proteins cannot leave the cell if they are translated in the cytoplasm

  all proteins that are secreted out of the cell have to begin in the ER
• proteins csn be transported/made post-translational or co-translational
• post translational

  import of proteins into the nucleus, peroxisomes and mitochondria occur after translation on free ribosomes in cytosol
• Co translational

  import of proteins into ER occcure while still attached to the ribosomes
• 2 types of proteins synthesized in the ER

  1. transmembrane proteins
  2. soluble proteins
• transmembrane proteins are partially translocated across membrane and embedded into membrane
• some transmembrane proteins have a single transmembrane domain, others have many
• some transmembrane proteins function in ER, others delivered to other membrane associated organelles or plasma membrane
• soluble proteins are fully translocated across membrane and released into ER lumen
• soluble proteins are either secreted from cell or stay in lumen of an organelle
• all proteins destined for ER have ER signal sequence (leader peptide)
• ER signal peptide is recognized by signal recognition particles (SRP)
• signal recognition particle is a multi subunit complex (6 proteins) + RNA (ribonucleoprotein complex) that recognizes ER signal that contains > 8 non polar amino acids (as an alpha helix) which binds SRP receptor in ER membrane and large ribosomal subunit
• signal recognition particle (SRP) binds to the large ribosomal subunit and binds signal peptide as it emerges from the ribsome
• second site on SRP blocks translation from continuing (safety mechanism) until SRP binds to SRP receptor
• SRP ribosome complex binds SRP receptor no to the surface of the ER
• SRP ribsome receptor complex interacts with protein translocator (Sec61) embedded in the ER membrane
• cystic fibrosis and retinitis pigmentosa (RP) are results of errors in proper ER processing
• CFTR protein is a transmembrane protein mutated in cystic fibrosis (deltaF508)
• studies have shown that the CFTR F508 protein is functional
• in CF patients, CFTR F508 protein is made in ER, where it is retained (due to the mutation) and eventually targeted for degradation
• RP is the most common inherited form of retinal degradation due to loss of specialized rod photoreceptor neurons in the eye
• gene for opsin is mutated (P23H) in RP
• in RP patients, opsinP23H is misfolded int he ER and makes it sensitive to protease degradation
• the accumulation of unfoled, misfolded, insoluble or damaged proteins can induce the unfolded protein response (UPR)
• cells can respond to UPR by:

  1. inducing apoptosis if stress is excessive
  2. increase production of chaperones (e.g., GRP78) if minimal
• GRP78 has been shown. to increase breast cancer cells which protect cells from chemotherapy drugs
• constant ER stress (UPR) can lead to cell signals that increase inflammation and apoptosis (cell and tissue)
• disease that have UPR associations:

  1. diabetes: UPR related cell death kills insulin producing cells
  2. neurological: UPR stress leads to cell death and nerve cell apoptosis
  3. autoimmune: UPR stress in various cell types can increase inflammation
• UPR activates 3 different type sof signal pathways to enable the ER to better handle protein translation and folding during ER stress
• UPR sensor IRE1 is a transmembrane protein kinase with cytoplasmic kinase and RNAse domains
• for IRE1 UPR sensors binding of misfolded proteins in ER activates kinase and RNAase domains
• activation of IRE1 kinase and RNAase domains causes splicing of pre-mRNA that produces unique transcription factor (TF)
• TF in IRE1 turns on genes to expand ER, increase protein folding capacity, and increase protein degradation of misfolded proteins
• UPR sensor PERK is a transmembrane protein kinase that phosphorylates translation factors (inhibiting overall protein synthesis)
• UPR sensor ATF6 is a transmembrane transcription factor that is cleaved when activated in golgi and turns on genes that can increase protein folding
• 4 forms of intracellular trafficking:

  1. endocytosis
  2. exocytosis
  3. secretion
  4. transport
• Dr. George Palade Yale won the Nobel prize in medicine in 1974 for the existence of vesicle trafficking compartments